The invention relates to a process for the removal of sulfur dioxide from waste gas.
Gaseous effluents from oil-fired or coal-fired power stations and waste incinerators contain various amounts of sulfur dioxide (SO.sub.2) which are detrimental to man and the environment (acid rain).
Existing processes for the desulfurization of waste gases can be divided into wet processes, spray-dry processes and dry processes.
In wet processes, the waste gases are contacted with an aqueous solution or suspension wherein the sulfur dioxide is dissolved. In order to sufficiently bind the sulfur dioxide in the aqueous solution, the solution must be alkaline. Known processes use limestone, alkali or ammonia as the source of alkalinity.
Desulfurization of waste gases has often been effected by washing the sulfur dioxide from the gases using lime. This results in large amounts of gypsum, the utilization of which is becoming increasingly difficult since the quality requirements of gypsum become more stringent. In a process using ammonia (often referred to as ammonium sulfate process), sulfur dioxide is washed from the waste gas with an ammonia solution which eventually leads to the production of ammonium sulfate which may be used as a chemical fertilizer.
In a spray-dry process, a finely divided lime suspension is blown into an absorber, wherein the sulfur dioxide reacts with the lime to form a mixture of calcium sulfite and sulfate. The amount of water used is sufficiently low to avoid the production of waste water. However, the problem of disposing the mixture of calcium sulfite/calcium sulfate/fly ash has not been solved. Dry processes have not been used in practice, largely because of the very high costs. Such processes produce a concentrated gaseous flow of sulfur dioxide.
An important alternative process for the processes mentioned above is the reduction of the sulfur dioxide by biological processes. Biological reactors have advantages in that they use a relatively simple technology under moderate pressure and temperature conditions.